A variety of electronic devices such as laptop computers, personal digital assistants, cell phones, etc. may be powered by a battery. The battery may be a rechargeable battery of a variety of types such as a nickel cadmium, nickel metal hydride, or lithium ion. Such rechargeable batteries may be recharged whenever a suitable DC power source, e.g., an AC/DC adapter, is coupled to the electronic device.
It is desirable for a user to be able to monitor the charge on the battery to know when recharging of the rechargeable battery is necessary. There are a variety of battery gas gauge devices known in the art for estimating the remaining capacity of the battery based on a variety of monitored conditions. Such gas gauges typically present the remaining capacity of the battery as a percentage of some full capacity determination.
When a battery cell is manufactured its initial full capacity can be directly measured. Thereafter, charge flow into and out of the cell can be measured by appropriate Coulomb counting circuitry. Unfortunately, factors such as cell aging may cause the full capacity level at some future time interval to be less than the initial full capacity level. Further factors such as cell self discharge may cause the current capacity level to become increasingly less accurate as time passes.
One typical solution to these accumulated inaccuracies is to discharge a cell to a point near a discharge cutoff voltage level, Vdc, (discharging below the Vdc level may damage the cell). When the cell is discharged to this point, the estimated remaining capacity (Cer) can be inferred from a voltage measurement at time t measured across the cell terminals at the current charge/discharge rate. If the cell is then charged without interruption to full capacity, the estimated remaining capacity, as previously determined, plus the charge accumulated during the charge cycle as measured by Coulomb counting for instance, equals the new full capacity. For example, at time t, the voltage may be 3.0 volts at a terminal discharge rate of about 2A. This data may then by used to index a lookup table to yield estimated remaining capacity at that point in time, e.g., 0.2 Ampere-hour (AH) for a 2 AH cell. Coulomb counting during charging may then show an accumulated charge of 1.75 AH so the full capacity determination would equal 0.2 AH plus 1.75 AH or 1.95 AH. This is sometimes referred to as a “qualified discharge/charge cycle.”
However, this approach has several drawbacks. First, the estimated remaining capacity Cer inferred from the voltage measurement at the end of discharge may be considerably in error. This is because it depends on the behavior of the effective internal resistance of the cell, which can be difficult to predict in a practical gas gauge design. Second, this approach requires that the cell be discharged to a near empty state before measurement begins for Coulomb counting. If this does not occur within several cycles of normal usage, then the gas gauge may be required to request a “conditioning cycle” (forced qualified discharge cycle) to restore the gas gauge to the required accuracy. This requirement may add hardware cost and create user inconvenience. Third, the user might not be willing to wait for the cell to reach a completely charged state. If the user terminates the charge prematurely, the final charge capacity can be estimated but this introduces a source of error.